MENDELIAN INHERITANCE

Transcription

1 MENDELIAN INHERITANCE Medelian inheritance includes: 1. Mendelian principles (laws) 2. Gene interactions 3. Sex-linked traits 4. Genetic linkage 2016 Doc. MVDr. Eva Bártová, Ph.D.

2 2. GENE INTERACTIONS participation of two or more different genes in production of one phenotypic character (qualitative trait) cleavage ratio is changed in F2 generation!! which Mendelian condition is broken? RECIPROCAL INTERACTION interaction without change of cleavage ratio trait is present in more forms (phenotype); each of them encoded by one combination of parent alleles of genes e.g.: color of pepper F 2 : 9 R-C- 3R-cc 3 rrc- 1rrcc red brown yellow green 9 : 3 : 3 : 1

3 DOMINANT EPISTASIS dominant allele of one gene (epistatic) suppresses expression of dominant allele of second gene (hypostatic) e.g.1: color of dahlia F 2 : 9Y-I- 3Y-ii 3yyI- 1yyii yellow yellow ivory white 12 : 3 : 1 RECESSIVE EPISTASIS recessive homozygous constitution of one gene (epistatic) suppresses expression of dominant allele of second gene e.g.1: color of flowers of salvia (Salvia sp.) F 2 : 9A-P- : 3A-pp : 3aaP- : 1aapp violet white pink white 9 : 4 (white) : 3 (pink)

4 COMPLEMENTARITY dominant alleles of two (or more) genes cooperate in realization of phenotype trait is expressed only if at least one dominant allele of both genes is present at the same time e. g.: color of flowers of vetch (Lathyrus sp.) F 2 : 9C-R- : 3C-rr : 3ccR- : 1ccrr red white white white 9 (red) : 7 (white) COMPENSATION function of dominant alleles of two different genes is contradictory, their phenotype effects exclude each other e. g.: curvature of pea pod (Pisum sativum) F2: 10 : 3 : 3

5 INHIBITION dominant allele of inhibitor gene suppresses the manifestation of dominant allele of other gene but inhibitor gene itself has no effect on the phenotype!! e. g. color of feather of hen F 2 : 9C-I- : 3C-ii : 3ccI- : 1ccii white : red : white : white 13 (white) : 3 (red) DUPLICITY (MULTIPLICITY) in genes with the same effect on phenotype intensity of effect depends on if effects of genes cumulate or not and if there is relationship of dominance between alleles of particular gene very often minor genes (polygenes) - dominant alleles are identical (marked with same letter)

6 1) Duplicity noncumulative with dominance there is not cumulation of dominant alleles (yes or no) e.g.: shape of capsule in toothwort (Capsella bursa-pastoris L.) F2: ( ) : ( ) 15 : 1 2) Duplicity cumulative with dominance (complete dominance) intenzity is amplified by the number of dominant alleles e.g.: color of barley grain F2: darkbrown : sorrel : white 9 : 6 : 1 3) Duplicity cumulative without dominance (incomplete dominance) intenzity depends on total number of active alleles e.g.: color of wheat grain F2: dark red : red : bright red : pink : white 1 : 4 : 6 : 4 : 1

7 How cleavage ratios change in F2 generation? A-B- A-bb aab- aabb reciprocal interaction dominant epistasis recessive epistasis complementarity 9 7 compensation inhibition 13 3 duplicity noncumulative 15 1 duplicity cumulative with dominance duplicity cumul. without dominance AABB AaBB AABb AAbb aabb AaBb Aabb aabb aabb

8 3. SEX-LINKED TRAITS genes are located on gonosoms which Mendelian principle is broken? which condition is broken? 1) complete sex-linked - genes are located on heterologous regions of sex chromosome (crossing-over is impossible) 2) incomplete sex-linked - genes are located on homologous (pseudoautosomal) regions of sex chromosome (crossing-over is possible, but often blocked) heterologous homologous heterologous homologous

9 Complete sex-linked - genes are located on heterologous regions of sex chromosome a) gene is located on gonosome Y (holandric inheritance) - trait is inherited from father to son (hypertrichosis auriculae - hair in ears) b) gene is located on gonosome X (hemophilia or daltonism in human, color of eyes in fruit fly) uniformity of phenotype in F1 generation, if dominant allele is on X chromosome in female cross inheritance, if dominant allele is on X chromosome in male For explanation use the examples from practical (colour of eyes in fruit fly, haemophilia, daltonism..) hemizygous What is hemizygous?

10 Recessively inherited diseases: Haemophilia, daltonism

11 SEX-LIMITED TRAITS genes are located on autosome (somatic) of both sex trait is expressed only in one sex because of anatomy (cryptorchism in male = absence of one or both testes in scrotum) SEX-CONTROLLED TRAITS genes are located on autosome of both sex trait is expressed only in one sex phenotype of dominant homozygote and heterozygote is controlled by sex hormones (secondary sex traits - beard in man, size of fins in fish)

12 SEX-INFLUENCED TRAITS genes are located on autosome of both sex trait is expressed in both sex phenotype of heterozygote is influenced with sex of carrier - it is due to the presence of male or female sex hormones (color of ayrshir cattle, hairlessness in man)

13 4. GENETIC LINKAGE two (or more) genes are located on single chromosome and are genetically linked which Mendelian principle is broken? which Mendelian condition is broken? MORGAN S PRINCIPLES (LAWS)!!! 1. Genes are located on one chromosome linearly running subsequently 2. Number of linkage groups equals to number of pairs of homologous chromosomes How many linkage groups can have a human?

14 Thomas Hunt MORGAN ( ) American geneticist and embryologist studied mutations in the fruit fly (Drosophila melanogaster) demonstrated that genes are carried on chromosomes and are mechanical basis of heredity awarded the Nobel Prize in Physiology or Medicine in 1933 Drosophila became a major model organism in genetics Centimorgan = unit of genetic distance of genes on chromosome Centimorgan is now more commonly called map unit (mu) or locus map unit (LMU)

15 How many different gamets (and what genotype) are produced by individuum with genotype AaBb? How many different gamets (and what genotype) are produced by heterozygote AaBb, if there is genetic linkage between genes A and B? (it depends if it is cis phase or trans phase) cis phase trans phase AB ab A B a b Ab ab A b a B

16 POWER OF LINKAGE probability of crossing-over between alleles of different genes that are in linkage Complete linkage - small distance between genes, strong linkage and small probability of crossing-over Incomplete linkage - large distance between genes, weak linkage and large probability of crossing-over BATESON NUMBER (c) number of offspring with nonrecombinant phenotype c number of offspring with recombinant phenotype MORGAN NUMBER (p) 100 x number of offspringwith recombinant phenotype p (cm) number of all offspring Relations: p c p 100 p c 1

17 TESTING CROSSING similar to back crossing (crossing of heterozygote with homozygote), is used to find frequence of genotypes according to phenotype ratio in offspring Three-point cross (see handbook) interactions between three genes are observed is used for setting of chromosome map = order of genes and their distance in centimorgans (cm) Chromosome map: /posters/chromosome/chooser.shtml